Do not expect to be able to change level in oceanic airspace, the separations involved are just to enormous. It is strongly recommended that you ask for your clearance at the highest level you can possibly achieve on entering Oceanic airspace, as you burn off fuel the aircraft will be at optimum flight level about half way across.
You should not expect to get initial descent for your destination while within oceanic airspace. Domestic (radar equipped) airspace starts far enough away from all destinations that this is not necessary. Descents in oceanic airspace will only be given in the event of an in flight emergency such as engine failure, decompression, or for separation issues.
For all altitude changes, either climbs or descents, pilots should report “reaching” the new level/cruising altitude to ATC.